Hey Brianne,
I posted a description of what it was like to tell my parents in my blog. I did NOT want to tell them in a letter. It may have been easier, but I had the financial means to get on the flight and visit them and tell them. It was probably the hardest thing I've ever had to do. In my case, it worked out well. I talked to each of them individually, my mom when my dad was messing around outside (then again when we went grocery shopping and a few more times that day) then my dad when my mom was at work. I highly recommend making sure you have enough time to talk to them when you bring it up though, don't make it like a fit-it-in-when-you-can kind of thing. That's where a letter is nice because they can read, reread, etc., which may or may not be good.
I was always close to my mom so I think that I felt it would be ok before I ever did it. She kept saying "why didn't you tell me when you were younger?" but I think she understands. She just said she wants me to be happy and not miserable and she'll support me no matter what. My dad pretty much said the same thing. Who knows what the future brings, but as of now, I have the love and support of them. Just my experience. I've just started therapy, not on HRT or anything yet.
The reason I told them so early is that my feelings aren't going to change. I was told to wait to tell them until I knew 100% that I was going to go through with HRT, had a date, etc. I felt a lot differently about it. I've held this inside for too long (I'm 38 now) and I have to be honest with them. Not being honest with them was eating me just like the gender thoughts were, why add to the problem. I was thinking "maybe they can be a part of my support group?"
Good luck, let us know what you decide and how it goes. If you have any more questions or anything, just let me know 🙂 Meghan